The present invention relates to colour sequential illumination systems and has particular, though not exclusive relevance to such systems as applied to liquid crystal displays.
Colour sequential illumination systems are known which, for example, are utilised for the backlighting of liquid crystal displays. In such applications, the systems provide electronically controlled sequential pulses of light of the three primary colours which are integrated to provide a broad spectrum of perceived coulours when viewed by an observer.
The light sources in such systems are usually low pressure mercury/rare gas discharge lamps coated on the inside wall with a phosphor (an example of such a type of lamp is the conventional fluorescent lamp). The lamps are switched by a high frequency (&gt;10 kHz) supply and, hence, the rise and decay times of the phosphors used in the lamps have a profound effect on the colour quality of the light source. For example, if the green phosphor has a relatively long decay time then green light emission will continue for a signicant time after the green lamp has been switched off. Such continuing emissions after deenergisation degrade the colour quality of the subsequent emissions (containing only red and/or blue light) occurring after the extinction of the green lamp because light emission will no longer be synchronised with the pulses used to generate each colour. As a result the range and purity of colours which may be produced by such systems will be impaired.
The phosphors used in red and green lamps typically possess relatively long optical decay times for example, 0.59 ms and 0.78 ms respectively, as compared with that of a blue phosphor which, typically, is around 0.04 ms.